The present invention relates to voltage controlled oscillators and to a method and circuit for controlling voltage controlled oscillators.
The voltage controlled oscillator (VCO) is a key element in successful broadband receivers, particularly in the receiver front end where noise can have a dramatic effect upon the quality of a broadband signal coming from an antenna or digital TV cable. Careful VCO design is needed to cope with tuning over a wide range of frequencies for broadband requirements.
Radio frequency (RF) circuits have traditionally been implemented using integrated bipolar technology with external passive components. More recently however, RF circuits implemented using CMOS technology have been shown to provide significant cost savings compared to bipolar technologies. FIG. 1 illustrates a basic VCO design using a pair of CMOS MOSFET transistors M1 and M2 and a pair of varactor fine tuning elements CV1 and CV2. The varactors and fine tuning capacitors combine with the fixed value inductor L1 to produce oscillation at a frequency given by:
ƒo=xc2xd*pi*sqrt(LC) 
Control of the varactor capacitance and hence the frequency is achieved by varying the voltage applied to the tune xe2x80x9cnodexe2x80x9d. FIG. 2 illustrates the relationship between the voltage applied to the tune node, Tune(v), and the VCO oscillating frequency (where the modulation index is the slope in Mhz/v).
So-called accumulation mode MOSFET varactors (AMOS) can achieve a cmax to cmin ratio of 3:1, within the xe2x80x9cheadroomxe2x80x9d allowed by the on-chip supply voltages, and can offer a better Q value than other on-chip MOS devices. One approach to extending the tuning range is to introduce off-chip varactors, effectively removing the voltage headroom limit present on-chip. If off-chip varactors are not desirable, then an alternative approach is to utilise both fine and coarse on-chip tuning elements to extend the tuning range and resolution. FIG. 3 illustrates a CMOS circuit making use of this approach. A series of capacitors are switchable into and out of the VCO circuit, to supplement the oscillator tank, by turning on and off respective switches. FIG. 3 illustrates a circuit comprising four switched capacitors on each side of the tank, allowing up to sixteen channels with each channel having a level of fine tuning provided by the varactors connected to nodes tune and tuneb. The tuning characteristics of the circuit of FIG. 3 are illustrated in FIG. 4.
FIG. 5 illustrates schematically a closed loop synthesiser comprising a VCO. A controller writes an appropriate value to a register coupled to the VCO, the value being selected in dependence upon the frequency to be tuned to. The value held by the register is used to determine which capacitor elements will be switched into the VCO tuning circuit, and hence to provide an initial coarse tuning of the VCO. Inside the synthesiser the VCO output frequency is divided down by a value appropriate to the required frequency and compared to a reference frequency derived from a crystal oscillator source. The result of the comparison is converted into a form which adjusts the fine tuning of the VCO until the frequency and phase at the phase comparator inputs due to the VCO output and the crystal oscillator are identical. At this point a lock signal is generated to indicate locking.
Problems arise with this arrangement when fine tuning of the VCO causes the tuning voltage to go beyond the extremes of the tuning curve (e.g. as shown in FIG. 1). If lock is achieved too close to the extremes of the curve, any drift (due for example to temperature or supply voltage change) may cause loss of lock and an inability to retune.
According to a first aspect of the present invention there is provided a frequency synthesiser including:
a voltage controlled oscillator, VCO, having means for fine tuning the oscillator frequency and switchable capacitive elements for coarse tuning the oscillator frequency;
first comparison means for comparing the frequency and phase of the output of the VCO or a signal derived therefrom with that of a reference frequency signal to provide an error signal, the error signal being provided to the means for fine tuning the oscillator frequency;
second comparison means for comparing the error signal against one or more reference values; and
control means for receiving the result of the comparison from the second comparison means and for switching the switchable capacitive elements on and off in dependence upon the result so as to coarse tune the oscillator frequency.
By integrating control of the switching of the coarse tuning capacitive elements into the feedback control loop of the synthesiser, embodiments of the present invention reduce the risk that frequency lock will be achieved at the extremes of the fine tuning range of the VCO, or that lock cannot be achieved because the locking frequency lies outside of the available tuning range (surrounding the selected coarse tuning frequency). The reference signal or signals correspond to boundaries of the error signal beyond which the fine tuning means will not respond adequately.
Preferably, the means for fine tuning the VCO comprises a pair of varactors. The varactors are each arranged to receive the error signal.
Preferably, the switchable capacitive elements for coarse tuning the oscillator frequency each comprise a capacitor coupled in series with a transistor switch. The control means comprises a register for storing values defining the settings of the switchable capacitive elements (on or off). The register may be written to by a controller to define the initial settings of the capacitive switching elements. The control means may change the values stored in the register in dependence upon the result of the comparison between the error signal and the one or more reference values.
Preferably, the frequency synthesiser comprises a low pass filter for filtering the error signal. The low pass filtered error signal is provided to the first comparison means and to the means for fine tuning the oscillator frequency.
Preferably, the first comparison means is a phase locked loop, PLL.
Preferably, the second comparison means is arranged to compare the error signal against a pair of reference values corresponding to maximum and minimum error signal boundaries, the result indicating whether or not the error signal is greater than the maximum boundary, less than the minimum boundary, or between the maximum and minimum boundaries. In the event that the result indicates that the error is less than the minimum boundary, the control means will increase the capacitance in the VCO tuning circuit by switching on an additional capacitive element. In the event that the result indicates that the error is greater than the maximum boundary, the control means will decrease the capacitance in the VCO tuning circuit by switching off a capacitive element. In the event that the result indicates that the error is between the maximum and minimum boundaries, the settings of the capacitive elements will not be changed.
Preferably, the control means is arranged to act upon the result only after a predefined delay following the switching of one or more capacitive elements. This delay allows the system the stabilise before further tuning is carried out.
Preferably, the first comparison means comprises means for providing a lock signal at an output thereof, to indicate whether or not the frequency and phase of the output of the VCO is locked to that of the reference frequency signal or the derived signal. More preferably, the lock signal is provided to the control means. The control means is arranged to respond to an indication by the lock signal that lock has been achieved, by inhibiting further switching of the switchable capacitive elements.
According to a second aspect of the present invention there is provided a method of controlling a voltage controlled oscillator, VCO, of a frequency synthesiser, the method comprising the steps of:
comparing the output of the VCO or a signal derived therefrom against a reference frequency signal, and providing an error signal indicative of the frequency and phase difference between the two signals;
applying the error signal to control fine tuning varactors of the VCO for fine tuning the oscillator frequency;
comparing the error signal against one or more reference signals to provide a switching signal or signals; and
using the switching signal(s) to control switchable capacitive elements for coarse tuning the oscillator frequency.